[Paper Review] Late Reheating of the IGM by Quasars: A Radiation Hydrodynamical Simulation of Helium II Reionization

P. Paschos, Michael L. Norman+2 more|ArXiv.org|Nov 13, 2007

Galaxies: Formation, Evolution, Phenomena1 references10 citations

TL;DR

This study uses radiation hydrodynamical simulations to model helium II reionization by quasars, incorporating opacity effects in photoheating to accurately predict IGM temperature and Lyα forest properties. It finds that opacity increases the mean IGM temperature by a factor of ~1.7 compared to optically thin approximations, with He II absorbers showing broader lines consistent with physical extension.

ABSTRACT

We study the ionization and thermal evolution of the intergalactic medium during the epoch of \heii reionization by means of radiation hydrodynamical cosmological simulations. We post-process baryonic density fields from a standard optically-thin IGM simulation with a homogeneous galaxy-dominated UV background (UVB) which reionizes \hi and \hei at z=6.5 but does not have any contribution to the ionization of \heii. Quasars with luminosities proportional to the mass of the host halos are then introduced as point sources throughout the 100 Mpc simulation volume consistent with the Pei luminosity function. We evolve the spatial distribution of the \heii ionizing radiation field using a time-implicit variable tensor Eddington factor radiative transfer scheme. Simultaneously, we also solve for the local ionization of \heii to \heii and the associated photoheating of the gas. We find that the percolation of the \heiii regions is essentially complete by z=2.5. When comparing to a self-consistent optically thin simulation we find that in optically thick calculation the gas temperature is higher by a factor of approximately 1.7 at the mean gas density level. We use 300 random lines of sight to compute at $\bar{z} = 2.5 \pm 0.1$ a mean \heii \lya line transmission of $\bar{F} = 0.304 \pm 0.002$. We compare the broadening width of the \hi and \heii \lya lines to the results from the self-consistent optically thin simulation and find a shift by approximately 1.25 km/s of the b-parameter distribution. Estimating the relative broadening width between the two forests shows that the \heii median b-parameter is about 0.8 times the median \hi broadening width. This implies that the \heii absorbers are physically extended consistent with conclusions from observed lines of sight.

Motivation & Objective

• To investigate the thermal and ionization evolution of the IGM during He II reionization using realistic radiation transfer.
• To quantify the impact of opacity effects on photoheating rates during He II reionization, which are often neglected in standard simulations.
• To compare simulated He II Lyα forest properties with observations, focusing on line transmission and b-parameter broadening.
• To assess the validity of the optically thin approximation in modeling IGM temperature and ionization state during late helium reionization.

Proposed method

• Radiation hydrodynamical cosmological simulations are performed in a 100 Mpc box with baryonic density fields post-processed with a quasar-driven ionizing radiation field.
• Quasars are modeled as point sources at CDM density peaks, with luminosity proportional to halo mass and a power-law spectrum $J(\nu) \propto \nu^{-1.8}$.
• A time-implicit variable tensor Eddington factor radiative transfer scheme computes the spatial distribution of He II ionizing radiation at 4, 8, and 16 Ryd.
• Local ionization of He II and associated photoheating are solved simultaneously, including opacity effects via the full He II cross section approximation $\sigma_{\text{HeII}} \propto (h\nu/h\nu_3)^{-3}$.
• Synthetic absorption line spectra are generated from 300 random lines of sight to compute statistical properties of the He II Lyα forest at $\bar{z} = 2.5 \pm 0.1$.
• The b-parameter (Doppler broadening) of H I and He II lines is compared between the optically thick and thin simulations to assess thermal effects.

Experimental results

Research questions

• RQ1 How does including opacity in He II photoheating affect the IGM temperature during reionization compared to the standard optically thin approximation?
• RQ2 What is the impact of quasar-driven He II reionization on the statistical properties of the He II Lyα forest, such as mean transmission and line broadening?
• RQ3 To what extent do opacity effects alter the relationship between gas temperature and density during He II reionization?
• RQ4 How do the simulated b-parameter values of H I and He II Lyα lines compare with observations, and what does this imply about absorber structure?

Key findings

• The percolation of He III regions is essentially complete by $z = 2.5$, indicating that He II reionization is largely finished by this redshift.
• Including opacity effects increases the IGM temperature at mean density by a factor of approximately 1.7 compared to the optically thin simulation.
• The mean He II Lyα line transmission at $\bar{z} = 2.5 \pm 0.1$ is $\bar{F} = 0.304 \pm 0.002$, with a 11% variance across sightlines.
• The b-parameter for both H I and He II lines is increased by approximately 1.25 km/s due to opacity effects, indicating higher thermal broadening.
• The median b-parameter of He II absorbers is about 0.8 times that of H I absorbers, implying that He II absorbers are physically more extended than H I absorbers.
• The study confirms that the optically thin approximation underestimates the IGM temperature during He II reionization by a factor of ~2, consistent with prior theoretical work.